linux-kernel - Re: [RFC PATCH 00/11] printk: safe printing in NMI context

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Date:	Wed, 18 Jun 2014 14:37:52 -0700
From:	"Paul E. McKenney" <paulmck@...ux.vnet.ibm.com>
To:	Jiri Kosina <jkosina@...e.cz>
Cc:	Linus Torvalds <torvalds@...ux-foundation.org>,
	Linux Kernel Mailing List <linux-kernel@...r.kernel.org>,
	Michal Hocko <mhocko@...e.cz>, Jan Kara <jack@...e.cz>,
	Frederic Weisbecker <fweisbec@...il.com>,
	Steven Rostedt <rostedt@...dmis.org>,
	Dave Anderson <anderson@...hat.com>,
	Andrew Morton <akpm@...ux-foundation.org>,
	Petr Mladek <pmladek@...e.cz>, Kay Sievers <kay@...y.org>
Subject: Re: [RFC PATCH 00/11] printk: safe printing in NMI context

On Wed, Jun 18, 2014 at 11:32:53PM +0200, Jiri Kosina wrote:
> On Wed, 18 Jun 2014, Paul E. McKenney wrote:
> 
> > > I agree that it might work nicely for RCU stall detector indeed. I was 
> > > looking for solution that'd work nicely both for RCU and for sysrq-l 
> > > (where we can't rely on processess being stuck in any way).
> > 
> > Agreed.  And if some more generally useful approach appears, I will be
> > quite happy to adjust RCU to use it.  In the meantime, I expect that
> > my patch will be helpful.
> 
> Agreed. And we'll look into fixing sysrq-l in parallel I guess; once there 
> is a working solution (hangs with sysrq-l can be trivially reproduced 
> almost immediately), we can then migrate RCU to it.
> 
> Still, I feel bad about the fact that we are now hostages of our printk() 
> implementation, which doesn't allow for any fixes/improvements. Having the 
> possibility to printk() from NMI would be nice and more robust ... 
> otherwise, we'll be getting people trying to do it in the future over and 
> over again, even if we now get rid of it at once.

Well, we could always have printk() splat if invoke in_nmi().

Oh, wait...  ;-)

More seriously, an in_nmi() printk() could taint the kernel, set a
flag that results in a deferred splat, do a trace_printk(), or any
number of things to let the developer know that this was a bad idea.

							Thanx, Paul

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